1. Field of the Invention
This invention relates to the preparation of emulsions and more particularly it relates to apparatus for the continuous preparation of high internal phase ratio emulsions.
2. Description of the Prior Art
Various apparatus have been known and employed for the preparation of emulsions, and in the majority of applications, the apparatus was adapted to the preparation of emulsions only in batch unit processing. Some apparatus was proposed for the continuous preparation of emulsions. In such apparatus, the proportioning of the internal and external phases required precise metering devices, such as positive displacement metering pumps, and sophisticated controls to insure that these pumps delivered the proper ratio of internal and external phases. Then, the two precisely metered internal and external phases were passed to some type of a premixing device which produced a loose dispersion of the internal phase within the external phase. Then, a suitable pump was employed to force the loose dispersion through a mixer which provided the necessary shear for reducing the particle sizes of the internal phase within a continuous external phase, so that the dispersed phase was in droplet sizes of less than a few dozen microns so that a stable emulsion was produced. The mixing device employed elements which could be energized in a fashion that fluids passed between them were commuted to a finally divided state. The mixers can be generally classified as devices capable of producing high shearing, such as high speed impellers, colloidal mills, ultrasonic dispersers, and like machines where the internal phase was subject to high shearing action to be dispersed within the external phase.
Emulsions could be made continuously in such equipment but none of these known apparatus for producing emulsions can produce an emulsion with an internal-external phase ratio in excess of 75 to 25% by volume. First, the properties of these high internal phase ratio emulsions include non-Newtonian flow properties, thixotropic characteristics, and a high sensitivity to shear magnitudes which, if they exceed the stability point of the desired emulsion, cause an inversion between the internal and external phases. For example, emulsions containing 10 to 30% by volume internal phase can be made easily on a continuous or semi-continuous basis. However, as the amount of internal phase begins to rise above the 75% level, the unusual properties of the high internal phase ratio emulsions prevent the ready application of known apparatus for their preparation. One unusual property is the reduction of the effective viscosity of emulsified composition to approximately that of the external phase upon the application of shear. As this condition is approached, the viscosity of the emulsion composition changes rapidly, and this result rapidly changes the flow of the ingredient internal and external phase materials through the various metering and pumping devices. The pumping means may suddenly encounter a change of fluid condition from a relatively low viscosity to a very high viscosity and then suddenly to a reduced viscosity as the desired high internal phase ratio emulsion is obtained. This viscosity changing effect is not the actual viscosity as would be taken in a Fann viscosity meter, but rather is the apparent effective viscosity of the emulsified mass within the system. Reference may be taken to U.S. Pat. No. 3,565,817, where there is a detailed description of the composition preparation and properties of high internal phase ratio emulsions wherein the internal phase is present in excess of 75% by volume of the resultant emulsion. The description of this patent is incorporated herein, along with the additional references listed therein which describe the ingredients and desired emulsions having internal and external phase ratios above 75 to 25% by volume.
The great complexity of suitable known equipment for producing on a continuous basis the high internal phase ratio emulsions may be readily appreciated by reference to the movement and control of the internal and external phase flow streams accomplished by the use of positive displacement rotary vane or gear pumps separately driven and the proportion of the internal phase to external phase controlled by an electronic two component blender which maintains the phase ratio either by adjusting the pump delivery rate or by changing the pump motor speed through an SCR controller or by adjusting a variable volume pump control or maintaining the phase ratio by properly positioning flow control valves in the process stream flow lines. The use of such electronic control requires selective and expensive metering devices to indicate the flow rate to the controller. All of this equipment accomplishes the task of providing a variable volume and controlled proportion of the two liquid streams. None of this equipment is used in producing shear to form the emulsion. Separate emulsifying equipment is provided to mix and emulsify the two streams and thereby produce the desired emulsion. Apparatus of this nature is extremely complicated and demands the utmost in process control features, so that the necessary critical proportioning of the ingredients of the high internal phase ratio emulsion can be obtained. In addition, the actual operation of the mixer providing the necessary shear for producing the emulsion establishes another critical operating parameter in known apparatus. Other apparatus difficulties with these known devices can be readily appreciated by reference to the preceding reference material.
High internal phase ratio emulsions possess peculiar rheological properties. When subjected to sufficiently low rates of shear, they behave like elastic solids. As the rate of shear is increased, a point is reached where they begin to flow. This is referred to as the "yield value". When such emulsions are subjected to increasingly higher rates of shear, they exhibit non-Newtonian behavior, and the effective viscosity decreases rapidly until shear rates in the range of 3,000 to 8,000 reciprocal seconds are reached. In this range the effective viscosity plateaus at a figure close to the viscosity of the external phase. As increasingly higher rates of shear are applied, a point is reached where the emulsifying agents can no longer maintain stable films, and at this point the emulsion breaks and cannot be reconstituted readily. The yield value and shear stability point, as well as the shape of the viscosity versus shear curve, will vary with each particular emulsion formulation.
Thus, in order successfully to prepare high internal phase ratio emulsions continuously, it is necessary to employ an apparatus capable of exerting sufficient shear upon the emulsions so that its viscosity approaches that of the phases, particularly that of the external phase, without causing the emulsion to break. While the apparatus maintains the high internal phase ratio emulsion in this state, the phases are introduced therein so as to produce the emulsion on a continuous basis.